It has become relatively common within the last few years to drill wells in the search for oil and gas and the like with a portion of the wellbore deviating from the usual vertical orientation. The deviation may extend for a considerable distance at a substantial angle from the horizontal and then return to the usual vertical orientation. In drilling such wells, a device known as a whip stock is set at spaced intervals along the wellbore as the drilling progresses to cause the wellbore to deviate from the vertical until the desired, relatively horizontal deviation angle is attained. The wellbore is then drilled for as much as several thousand feet along the deviation angle and may be returned to the vertical orientation by setting the whip stock at spaced intervals as previously mentioned.
As is well known in the art of drilling wells, there are many well tools including such as well logging tools that are generally run into the wellbore on a wireline and/or cable to perform various operations therein. Such tools depend upon the force of gravity to permit positioning of the well tools at the desired formation in the wellbore.
Logging for vertical wells is performed using steel wireline cables to transport the logging tools. The weight of the tool forces the tool and line down to the bottom of the hole. In deep horizontal wells and in highly deviated wells, the force vector component pushing the tool down the hole is insufficient to overcome the frictional forces of the tool and line rubbing on the walls of the hole and alternate methods must be used. One of the current methods for logging deviated and horizontal wells is to use the drill pipe to transport the logging tool. This method however is time consuming and expensive. In addition, the high mass of magnetic material can interfere with some logging instruments.
In recent years, coiled steel tubing has been used to log horizontal holes and deviated wells. Steel tubing is limited to diameters on the order of 1.5, inches and wall thicknesses to around 0.1 inches in order to permit spooling. This small size limits the available bending stiffness to resist buckling and tensile strength required to pull the tubing out of the hole. The tensile strength limitation establishes a critical depth for the steel tubing beyond which it cannot be used since the weight of the tubing exceeds its own strength. The strength factor limitation prohibits coiled tubing from logging some extended reach wells.
It has become essential to provide some means of forcing wireline actuated tools through horizontal wells and highly deviated wells particularly when such wells are of substantial depth.
Present operations using coiled tubing for logging operations utilize an electrical logging cable inserted within the bore of the coiled tubing. When real time data is not required, a logging tool with downhole memory is used. When real time data is required, a dedicated reel of coiled tubing is used with the wireline permanently installed in the tubing. This takes up substantial cross-sectional space within the tubing and thus renders the coiled tubing unsuitable for other operations requiring a flow path or open bore through the tubing. Conducting fluids from the surface to a downhole location through the tubing may be used to provide means for treating formations or for operating a mud motor to drill through the formations. In addition, it may be desirable to pump devices through the coiled tubing bore to a downhole location for various operations. In any event, an open bore within the coiled tubing is essential for many operations and for this reason it is desirable to not have electrical conductors or the like positioned within the open bore portion of the tubing.